The invention relates to an apparatus for thin-coating processes for treating substrates of large surface area, typically with substrate dimensions between 500-1,000 mm by 500-1,000 mm, using a plasma which is excited by high-frequency electromagnetic waves, especially microwaves, and the electromagnetic waves from a source are coupled by means of a waveguide, consisting of at least one hollow conductor as well as a horn antenna, into a plasma chamber which is separated from the waveguide by a window permeable to electromagnetic waves.
In numerous fields of technology plasma processes are used both in coating and in etching apparatus. Various forms of electrical energy serve to produce a plasma. For example, it is possible to use direct currents or low-frequency alternating currents for the production of plasmas. Particularly advantageous is the production of plasma by high-frequency electromagnetic waves, especially microwaves, because in this case, among other benefits, no electrodes are needed which can be contaminated and ablated, and because the plasma produced by microwaves has a higher density of ions and electrons and therefore can be held at higher pressure than a plasma produced by other methods.
The waves emitted by a microwave source with a frequency of, e.g., 2.45 GHz and a free space wavelength of about 12 cm, are usually carried by means of a waveguide and coupled into a plasma chamber, the waveguide consisting as a rule of a hollow conductor and an antenna structure with coupling elements or a horn antenna.
An apparatus is known, for example (OS DE 37 05 666) which is suitable for producing a plasma and treating substrates therein. In this apparatus a microwave guide with a horn radiator (=horn antenna) is used, and rotatable metal reflectors are used within the horn radiator and the plasma chamber to influence the field distribution. The great number of standing waves thereby produced are also referred to as a multi-mode system. By this superimposition of many individual vibrations a wave field is formed which has an improved homogeneity with respect to the microwave lobe radiated by the horn radiator.
For the treatment of substrates of large area, with substrate dimensions, for example, up to 1,000 by 1,000 mm, however, the requirements as to the homogeneity of the plasma on the entire substrate surface have been further increased. Thus it is to be found disadvantageously that the market requirements as to the homogeneity of the coatings that can be made with the above-described apparatus, insofar as coating thickness and coating composition are concerned, can no longer be satisfied.